Isolation of 6‑Deoxytetrodotoxin from the Pufferfish, Takifugu pardalis, and a Comparison of the Effects of the C‑6 and C‑11 Hydroxy Groups of Tetrodotoxin on Its Activity

Identification of new tetrodotoxin (TTX, 1) analogues would be significant in the elucidation of its biosynthetic pathway and a study of its structure–activity relationships. In this study, a new TTX analogue, 6-deoxyTTX (2), was isolated from the ovary of the pufferfish, Takifugu pardalis, and the structure was determined using spectroscopic methods. Compound 2 was also identified in other marine animals, Nassarius snail and blue-ringed octopuses, using LC-MS. Furthermore, we investigated the voltage-gated sodium channel blocking activity of 2 by examination of the inhibitory activities to cytotoxicity induced by ouabain and veratridine in mouse neuroblastoma cells (Neuro-2a). The activities were then compared with those of 1, 11-deoxyTTX (3), and 6,11-dideoxyTTX (4). The EC50 value for 2 was estimated to be 6.5 ± 2.2 nM, approximately 3-fold larger than that of 1 (2.1 ± 0.6 nM) and approximately 20-fold smaller than that of 3. These results suggested that contribution of the C-6 hydroxy group to the activity is less than that of the C-11 hydroxy group.